Need smoke to pour out of a container
Change the Border Collisions setting to Collide All. And to make the smoke fall naturally, change the Temp Diff to a negative value, the lower the value (i.e. farther below 0), the faster the smoke will fall. The Density value also affects this, but I honestly don't really understand that; I assumed a higher (i.e. higher above 0) density would make it heavier, but it seems a negative value actually makes it fall faster. In the image I've also enabled Dissolve, to prevent the smoke from completely filling up the domain, but I've set a high enough value to still have time to come out of the box and fall to the floor. I've also used very low values for density and temperature difference, just to see what happened. The exact values you'll need, depend on your setup.  

  

And here's the test animation, as promised. A bit grainy, a little jumpy and very "vanilla", but I tried to get it done fast rather than with high quality.